Both n-type chalcogenide compositions and/or p-type chalcogenide compositions have been incorporated into components of heterojunction photovoltaic devices. The p-type chalcogenide compositions have been used as the photovoltaic absorber region in these devices. Illustrative p-type, photovoltaically active chalcogenide compositions often include sulfides and/or selenides of at least one or more of aluminum (Al), copper (Cu), indium (In), and/or gallium (Ga). More typically at least two or even all three of Cu, In, and Ga are present. Such materials are referred to as CIS, CIAS, CISS, CIGS, and/or CIGSS compositions, or the like (collectively CIGS compositions hereinafter).
Absorbers based upon CIGS compositions offer several advantages. As one, these compositions have demonstrated high power conversion efficiencies both at laboratory scale and increasingly at manufacturing scale. Thin film solar cells incorporating CIGS-based absorber layers can be very thin while still capturing a very high percentage of incident light. Such solar cells, therefore, present a potential low cost alternative to more traditional silicon based technologies. For example, in many devices, CIGS-based absorber layers have a thickness in the range from about 1 μm to about 2 μm. This is in contrast to much thicker silicon-based absorbers. Silicon-based absorbers have a lower cross-section for light capture and generally must be much thicker to capture the same amount of incident light.
According to one proposed technique for manufacturing chalcogenide semiconductor films, deposition methods are used in an initial stage to deposit and/or co-deposit all or a portion of the desired chalcogenide semiconductor constituents in one or more layers to form precursor film(s). At least a portion and sometimes all of the chalcogen(s) might not be included in the precursor film(s) at this stage. Instead, all or a portion of the chalcogen content might be incorporated into the precursor via chalcogenization at a later processing stage. In many instances, chalcogenization involves selenization and/or sulfurization. Chalcogenization often involves a thermal treatment of the precursor film(s) in the presence of chalcogen(s). This kind of thermal treatment not only incorporates chalcogen into the precursor but also converts the crystal structure of the film(s) into a more suitable crystal form for photoactive functionality.
Chalcogenide semiconductors are useful in a wide range of microelectronic devices including photovoltaic devices. Photovoltaic devices also are referred to as solar cells. Solar cells based on a chalcogenide semiconductor such as Cu(In,Ga)Se2 achieve higher power conversion efficiency by incorporating alkali metal content, e.g., Na incorporation, into the semiconductor as is well known in the literature. This incorporation improves performance at least in part by passivation of defects at grain boundaries, influencing grain growth, and/or improving the p-doping of a thin chalcogenide semiconductor film.
In earlier research, when chalcogenide semiconductor solar cells were deposited on soda lime glass, it was discovered that Na diffused from the substrate and improved the device performance. Such diffusion was a relatively uncontrolled process. Accordingly, processes were developed in which films containing alkali metal content were purposefully deposited in combination with or prior to chalcogenide precursor films in order to provide a more controllable source of alkali metal to incorporate into the resultant semiconductor film.
In some instances, such alkali-metal containing films have been provided by evaporating Na containing salts such as Na2Se, Na2O, and/or NaF at temperature ranging from 600 C to 1200 C. The evaporation temperature has depended upon factors such as the specific material, thermal source designs, targeted film thickness, and deposition rate. In some cases, the material was in fact sublimed rather than evaporated depending on the pressure and temperature applied. Operation and control of NaF thermal sources can be very difficult when applied over large areas due to reasons such as source thermal uniformity, spitting, dependable deposition monitoring methods, and instability of thermal sources.
The use of sputtering techniques to incorporate sodium into chalcogenide semiconductors has been described in EP 2410556 A2; WO 2012054467 A2; US 2012/0217157 A1; and WO 2012147985 A1; and U.S. Pat. No. 5,994,163.
Sputtering offers the potential to provide much more uniform and controllable deposition of films. Many alkali metal compounds such as NaF, however, are not conductive. NaF, for instance, can be sputtered using RF sources, but the rate of deposition is low. Also, targets made from only such compounds lack thermal conductivity and durability and often crack. In addition, large power RF source are bulky, expensive and may not be as optimum for high throughput manufacturing as AC or DC sputtering strategies. This means that RF sputtering may not be as desirable for large scale manufacturing of chalcogenide semiconductor films that include alkali metal content. Accordingly, there is a need for improved sputtering methods that are used in the fabrication of chalcogenide semiconductors incorporating alkali metal-containing species.